A position determination of a cylinder piston which is as exact and as fast as possible is used for a number of technical applications. The exact piston position is in particular required as a control variable for the automated control of a piston-in-cylinder unit. In addition, the position determination can be used to satisfy operating safety since the inlet of the hydraulic medium, in particular of the hydraulic fluid, can be exactly controlled and consequently stopped in good time in extreme positions of the cylinder piston.
A precision position determination is above all of importance in the automatic control of the piston-in-cylinder unit in construction machinery or hoisting units. The piston-in-cylinder unit actuates the piece of working equipment of the construction machine or hoisting unit in a typical manner. A sufficiently exact position determination of the piston-in-cylinder unit increases the quality of the control and is therefore absolutely necessary. One of a plurality of techniques known in the prior art for the determination of the current position of a cylinder piston is the use of specifically arranged cable line potentiometers.
An alternative method for the measurement of the longitudinal extent of the hydraulic cylinder works in accordance with a magnetorestrictive principle. In this respect, however, the piston rod has to be drilled through over the total stroke length for the integration of the measurement element into the cylinder, which weakens the cylinder mechanically and causes high integration costs. This installation variant can no longer be used with large cylinders. The required cable conduit of the measurement electronics used within the cylinder likewise represents a challenging task.
A further method for the position determination of hydraulic cylinders and pneumatic cylinders is known from DE 10 2005 029 494 A1. The piston cylinder arrangement resulting from this has a sensor element which interacts with a magnetic transmitter element. The piston takes over the function of a plunger (transmitter element) whose position is detected by a path sensor coil (sensor element) and is determined by evaluation electronics. The path sensor coil is located in the side of the cylinder liner remote from the piston rod.
DE 195 37 877 A1 discloses an apparatus for the determination of a piston position in a cylinder using electromagnetic waves in which a conclusion is drawn on the position of the piston in the cylinder with the aid of a scalar evaluation.
A further measurement principle is known from DE 10 2011 088 381 A1. The resonance measurement method used therein determines the current position of the piston on the basis of the capacitance formed and of the inductance of the piston cylinder arrangement. The measured inductance is substantially decisive as the length-changing variable. It is, however, disadvantageous in the named measurement method that sporadically occurring short-circuit effects between the piston and the jacket surface, for example due to contamination within the piston-in-cylinder unit, can falsify the measurement accuracy. A residual risk thus remains on the operation of such cylinder units.
In addition, interference effects are present in the measurement of the cylinder impedance since external unstable impedance amounts cannot be separated from the useful cylinder impedance. Such interference amounts are inter alia the external coupling means which are required for measuring the impedance. The exact positioning suffers due to this, in particular with non-constant working conditions such as fluctuating temperatures, pressure and a mechanical strain on the cylinder.
It is therefore the aim of the present disclosure to provide the skilled person with an alternative or improved and more reliable measurement method as well as a measurement apparatus corresponding thereto for the position determination of a cylinder piston.
The apparatus for the position determination of a cylinder piston located in a cylinder liner accordingly comprises a port for coupling or for decoupling an electrical signal into or out of the interior of the cylinder liner, wherein a signal coupled into the interior of the cylinder liner interacts with the cylinder piston. The apparatus is characterized by a unit for determining the position of the cylinder piston which is connected to the port and which is configured to determine the position of the cylinder piston in the cylinder liner with the aid of a vectorial measurement of a signal decoupled from the cylinder liner.
It is possible by the provision of the port to measure a measurement path comprising a cylinder liner and a cylinder piston. In this respect, an excitation signal is first coupled into the interior of the cylinder via the port and, after a propagation of the signal in the interior of the cylinder liner, the signal is decoupled from the interior of the cylinder lie via the port and is determined with the aid of a vectorial measurement. The phase of the signal is determined in addition to the amplitude value in the vectorial measurement.
The vectorial measurement of the signal allows a more accurate and faster determination of the position of the piston in the cylinder liner.
The port in this respect describes every kind of coupling element which can transmit an electrical signal inductively, capacitively or ohmically to a conductive structure.
There is advantageously a first port for coupling an electrical signal into and a second port for decoupling an electrical signal out of the cylinder liner.
On the presence of two ports, with a first port being provided for coupling an electrical signal into and a second port for decoupling an electrical signal out of the cylinder liner, the unit for determining the position of the cylinder piston is configured in an advantageous modification of the present disclosure to detect at least one scattering parameter of the port with the aid of a multi-gate measurement and to determine the cylinder piston position on the basis of the at least one detected scattering parameter. What is innovative in this respect is the consideration of the cylinder as a multi-gate system and the recognition that a position of the piston in the cylinder liner is possible using the scattering parameter.
This simplifies and accelerates the measurement of the piston position and additionally allows the implementation using components readily available on the market. The unit for determining the position of the cylinder piston can thus, for example, comprise a commercial network analyzer.
This consideration can also be used if only one port is present which both couples a signal into and decouples a signal out of the cylinder liner. A signal can, for example, simultaneously be coupled into and decoupled out of the cylinder liner over only one port. However, this produces less exact results than on the implementation with two ports separate from one another.
In accordance with a further advantageous modification of the present disclosure, the apparatus for the position determination of a cylinder piston comprises a piston rod which is connected at one of its ends to the cylinder piston; a piston rod bearing which is arranged at a section of the piston rod; a first port for coupling an electrical signal into the interior of the cylinder liner and/or onto the piston rod; and a second port for decoupling an electrical signal out of the interior of the cylinder liner and/or from the piston rod. The unit for determining the position of the cylinder piston is furthermore configured to detect at least one scattering parameter of the ports with the aid of a multi-gate measurement and to determine the cylinder piston position on the basis of the at least one detected scattering parameter.
The cylinder piston whose position is to be determined by the apparatus in accordance with the present disclosure is therefore arranged at an end of the piston rod. The piston rod bearing typically serves the support of the piston rod and terminates a cylinder jacket at one side. The cylinder piston is in this respect displaceably received in the cylinder jacket. As a rule, the piston rod bearing is arranged outside a cylinder jacket or a cylinder liner, whereas the cylinder piston is arranged movably within the cylinder jacket. In a cross-sectional view extending in the longitudinal direction of the piston rod, the cylinder piston has a greater extent than the piston rod so that a termination is achieved with the inner surface of a cylinder jacket which is exact as possible.
The surfaces of the cylinder-in-piston unit, i.e. of the complete cylinder, can in this respect also be coated with electrically non-conductive materials; however, the bulk material has to be electrically conductive so that a current can flow at all. Only one conductive layer is also sufficient for this purpose as would be the case with a plastic cylinder having a thin metal coating.
The present disclosure is inter alia based on the basic recognition that the position information of the cylinder piston can be obtained from a vectorial measurement of the scattering parameters of a cylinder. The first port in this respect represents a first gate and the second port in this respect represents a second gate which interacts with the piston rod or with the piston rod bearing. It is, however, also possible only to provide one port which can be considered as a first port in the signal output direction and as a second port in the signal reception direction. The embodiment with two ports physically separated from one another is, however, of advantage since better measurement results can be achieved.
The hollow space between the piston rod and the cylinder liner serves as a propagation space for electromagnetic waves which are used for the measurement of the piston position. The electrically conductive parts of the cylinder represent boundary conditions for electromagnetic waves.
The transmission factor S21 or S12 between the first and second ports thus serves, for example, as a basis for the determination of the cylinder impedance so that a conclusion can then be drawn on the exact position of the piston.
The circumstance is advantageous with the position determination on the basis of at least one scattering parameter that the impedance contributions of coupling elements such as the ports can be suppressed by a clever choice of the measurement frequency. A more exact position determination of the piston thereby results. In addition, the presented disclosure achieves more exact results under non-constant working conditions such as fluctuating temperatures, pressure and mechanical strains on the cylinder.
In accordance with an advantageous modification of the present disclosure, the at least one scattering parameter which is used for determining the piston position is a transmission factor. With a dual gate, this is the scattering parameter S12 or S21 which are identical in an advantageous variation of the present disclosure. Accordingly, the dual gate formed with the first port and the second port is transmission-symmetrical.
The first port and the second port are advantageously arranged in a fixed position with respect to a stroke movement of the cylinder piston and of the piston rod. An arrangement at or in the piston rod bearing is particularly advantageous. The ports can in this respect in particular be fixedly connected to a cylinder jacket in which the cylinder piston is received. The position of the port or ports with respect to the cylinder liner does not change. The unit for determining the position of the cylinder piston is then configured to determine the cylinder piston position with respect to the ports arranged in a fixed position.
The impedance of the measurement path between the cylinder piston and the cylinder liner changes through a variation of the piston position. Signal changes at the port or ports can thereby be measured which are directly related to the stroke variation of the piston.
In accordance with a further advantageous modification of the present disclosure, the first port and/or the second port is/are a coupling element which allows a transmission of an electrical signal to the piston rod or from the piston rod in a capacitive, inductive and/or ohmic manner.
In a simple embodiment, the ports comprise electrical connectors (electrodes) which are in operative connection with the piston rod so that one port respectively corresponds to one gate of the multi-gate measurement. The second connector of a port belonging to the gate is in this respect represented by a common mass level. Suitable electrical connectors are all electrical connections which allow an interaction between an external coupling and a position-variable waveguide (in the form of the piston rod here).
It is possible to integrate the electrical connectors directly in the piston rod bearing to allow a signal transmission to the piston rod of the cylinder in an inductive, capacitive or ohmic manner. A capacitive or inductive connection is advantageous since the interaction with the piston rod is thus implemented free of wear without the changes of a contact caused by the service life resulting in a deterioration of the signal transmission. A direct contact of the piston rod with the ports is, however, likewise possible.
In an advantageous modification of the present disclosure, the first port (4) and the second port (5) can thus have at least one electrical connector comprising an electrode. This electrode is then typically connected to the piston rod and provides a bidirectional signal transmission of electrical signals.
The unit for determining the position of the cylinder piston of the apparatus is configured to carry out the multi-gate measurement of the ports for determining the at least one scattering parameter at least two mutually different measurement frequencies or measurement frequency ranges. The multi-gate measurement is a dual gate measurement with only two ports.
A specific frequency which is used in the determination of the scattering parameters results in a piece of position information of the cylinder piston which is unambiguous up to a multiple of the magnitude λ/2. This multiplicity can be resolved with the help of the measurement of the scattering parameters at a further frequency and the position determination can be carried out particularly reliably. In this respect, λ defines the wavelength and is related to the frequency through the inverse of the propagation speed of the electromagnetic wave in the medium.
In accordance with a further development of the present disclosure, the apparatus comprises a third port for the signal transmission of an electrical signal which interacts with the piston rods. The provision of a plurality of ports serves to increase the reliability of the cylinder piston position to be determined. The number of ports is accordingly not limited to two, but can adopt any desired large number. It is of advantage in this respect if a scattering parameter Si,j, where i≠j, that is a transmission factor, is used between two gates for determining the cylinder piston position.
The second port and/or the third port is/are formed by at least one electrical connector in the form of an electrode which interacts with the piston rod or is in contact therewith. Since each of the ports typically corresponds to a gate of the multi-gate measurement carried out by the unit for determining the position of the cylinder piston, a common mass level is used as the second port or second electrode belonging to the gate.
In accordance with a further development of the present disclosure, the piston rod bearing comprises two channel-shaped elements which each approximately have the basic shape of a half-ring in their cross-sections, and with each of them facing the concavely arched sides of the channel-shaped element of the piston rod. The piston rod is received in the two channel-shaped elements and is displaceably supported by them. The channel-shaped elements can also be considered as part-cylinder jacket surfaces which are obtained by a sectional plane extending in parallel with the longitudinal direction of the cylinder jacket.
The two channel-shaped elements receive the piston rod in their concave arches. In addition, a gap extending in the longitudinal direction of the piston rod is located between the mutually facing end faces of the two channel-shaped elements so that the two channel-shaped elements are not electrically connected or are not directly electrically connected to one another.
It is hereby ensured that a direct electrically conductive connection of the two ports only takes place via a connection extending over the piston rod and is not short-circuited by other elements such as the piston rod bearing.
In accordance with one embodiment, the first port is arranged at or in one of the two channel-shaped elements and the other port is arranged at or in the other one of the two channel-shaped elements.
The unit for determining the position of the cylinder piston can furthermore be configured, under the simplifying assumption that only TEM modes are taken into account in the hollow space between the piston rod and the inner wall of the cylinder liner, to determine the position of the cylinder piston with the help of the relationship
      e          i      ⁢                          ⁢      ω      ⁢                        2          ⁢          L                c              =            p      3        ⁡          (                                    p            2                                M            -                          p              1                                      +        1            )      where ω is the angular speed, L the length of the piston rod in the hollow space, c the speed of light in the cylinder medium, p1, p2, p3 are calibration parameters which can be determined via simulation or calibration and M is a signal measured at the ports. M is in this respect one of the transmission factors of the scattering parameters of the ports, for example S21 or S12. The calibration parameters p1, p2, p3 can be determined via a simulation or a calibration.
As can be seen from the equation, an unambiguous piece of position information can be determined for any desired frequency up to multiples of the parameter λ/2.
The unit for the determination of the position of the cylinder piston advantageously comprises a vectorial network analyzer to determine the scattering parameters.
It is advantageous with the apparatus in accordance with the present disclosure that the position information can be acquired at any desired measurement frequency, that is also at the frequencies at which the impedance contribution of coupling elements is negligible.
The present information furthermore describes a method for the position determination of a cylinder piston, wherein a first port and a second port are each present for the signal transmission of an electrical signal from and to a piston rod. The method is characterized in that a multi-gate measurement is carried out for obtaining at least one scattering parameter, including one of the transmission factors S21 or S12, of the ports (4, 5) and the at least one detected scattering parameter is used for the determination of the cylinder piston position.
The present disclosure changes the manner of consideration of the cylinder system in accordance with which it becomes clear that the complete physical information can be found in the vectorial response of the system, that is both in the amplitude and in the phase of the measured signal. This circumstance is taken into account by the determination of at least one scattering parameter.
The method is further developed in that the first port is arranged in a fixed position with respect to a stroke movement of the cylinder piston, at a cylinder jacket in which the cylinder piston is received and the cylinder piston position is determined with respect to the first port arranged in a fixed position.
In accordance with a further modification of the method, the multi-gate measurement of the at least one scattering parameter of the ports is carried out with at least two mutually different measurement frequencies or measurement frequency ranges.
It is also of advantage to carry out the multi-gate measurement of the scattering parameters of the ports at a measurement frequency at which the impedance contribution of the ports or other coupling elements is minimal or negligible.
This allows a more exact position determination of the cylinder piston since the interference effects of the impedance of coupling elements can be reduced.
The position of the cylinder piston is determined on the basis of the relationship shown in the following under the simplifying assumption that only TEM modes are considered in the hollow space between the piston rod and the inner wall of the cylinder liner:
            e              i        ⁢                                  ⁢        ω        ⁢                              2            ⁢            L                    c                      =                  p        3            ⁡              (                                            p              2                                      M              -                              p                1                                              +          1                )              ,where ω is the angular speed, L the length of the piston rod in the hollow space, c the speed of light in the cylinder medium, p1, p2, p3 are the calibration parameters which can be determined via simulation or calibration and M is a signal measured at the ports. M is in this respect one of the transmission factors of the scattering parameters of the ports, for example S21 or S12. The calibration parameters p1, p2, p3 can be determined via a simulation or a calibration.
The present disclosure furthermore relates to a construction machine or to a hoisting unit having an apparatus of one of the above-described embodiments.
The present disclosure also comprises a construction machine or a hoisting unit having means which are configured to carry out one of the above-described methods in accordance with the present disclosure.
Further advantages and particulars of the present disclosure will be explained in detail in the following with reference to the embodiments shown in the drawings.